Holographic storage systems are preferably equipped with a servo system to accurately control the position of the optical read/write system relative to a storage medium, e.g. a holographic optical disk. Such a servo system usually makes use of servo marks such as pits located on the storage medium.
It has been proposed to use different laser wavelengths for a servo beam and the object and reference beams. In this case a recording material which is only sensitive to light of one specific wavelength is employed. Light with a different wavelength can then be used for servoing etc. The servo marks are separated from the recording material of the holographic disk medium by a dichroic layer. Such a prior art solution is depicted in FIG. 1. The holographic disk medium 1 includes a recording layer 2, in which the holograms 6 are recorded, a mirror layer 3 for reflecting the servo beam 9, a dichroic mirror layer 5 for reflecting the object and reference beams 8, and two buffer layers 4 for separating the other layers 2, 3, 5. Servo marks 7 are arranged in the mirror layer 3. In the example of FIG. 1, the object and reference beams 8 and the servo beam 9 are combined by a beam splitter 10 and focused into the holographic disk medium 1 by a focusing lens 11.
A further prior art concept, which is disclosed in EP 1471507, uses the same layer for reflecting the servo beam and the object and reference beams. In this way the dichroic layer is no more required, which makes the manufacturing of the holographic disk medium significantly easier. According to this solution the beams are laterally separated. A narrow track/groove or narrow pits are used for the generation of focus and track information. A wide mirror or land area between the tracks/grooves or pits is provided as a mirror located in the Fourier plane of the object and reference beams. A recording structure on a holographic disk medium in accordance with this solution is depicted in FIG. 2. A band 13 with servo marks 7 is arranged between each band 12 of holograms 6. The holograms do not overlap with the servo marks. No radial/lateral shift multiplexing is applied. Depending on the size of the holograms 6, i.e. the Fourier image, large beam separations are required. This leads to large aberrations of the servo beam, in particular in case of objective lenses with high numerical aperture (NA). In addition, only tangential shift multiplexing can be used.
A further prior art concept is disclosed in US 2005/002311. According to this concept, a hologram recording medium has a hologram layer and a servo layer. The tracks in the servo layer are located at least partly below the holograms. In order to minimize the influence of the servo marks on the object and reference beams the track width in the servo layer is either much larger or much smaller than the diameter of the object and reference beams.